Electrical stimulation is one of a variety of methods, which have been used for at least the past twenty-five years to tenderize meat products. Electrical stimulation results in an acceleration of rigor mortis and a corresponding more rapid decrease of pH in the meat. Electrical stimulation influences the progress of post mortem biochemical processes in the muscle tissue of the slaughtered animal.
When an animal is alive, the muscle tissue operates under a condition that is called aerobic metabolism, which simply means that oxygen is available to the muscle tissue. When an animal dies, the muscle tissue goes into a state called anaerobic metabolism wherein oxygen is not available to the muscle tissue. Aerobic metabolism involves a process wherein the muscles utilize the sugar/glucose and burns it in order to create energy for use in relaxation and contraction of the muscles. The energy resulting from the sugar/glucose used by muscles in relaxation and contraction is further converted into water and carbon dioxide as long as the animal is breathing and oxygen is being received into the body. The water and carbon dioxide exits the body as waste. Under anaerobic metabolism conditions, the muscles utilize sugar/glucose to create energy for use in relaxation and contraction of muscles. Under anaerobic conditions, contractions occur by applying electrical stimulation to the carcass, and relaxation of muscles results from removal of the electrical stimulation. The energy used during muscle contraction and relaxation under anaerobic conditions is not further converted into water and carbon dioxide that exits the body as waste. That energy is shunted off as lactic acid, carrying a corresponding decline in muscle pH.
During anaerobic metabolism conditions, there is a gradual build up of lactic acid, resulting from the contraction and relaxation of muscles that causes the pH in the muscle to shift. Normal pH in living muscle tissue is about 7. As the typical onset of rigor mortis occurs and there is a build up of lactic acid, there is a shift in pH from about 7 down to about the 5.6 to 5.8 range. As long as there is sugar/glucose in the muscle tissue of the body that can be used as energy, the muscle will go through relaxation and contraction. When the sugar/glucose is depleted, the filaments present in the muscle tissue become fixed and rigid, thus the term "rigor mortis" refers to stiffening of the muscles after death.
Electrical stimulation of muscles accelerates the process of rigor mortis because electrical stimulation of the muscles causes severe contractions. The contractions in the muscles result in the muscles using up the sugar/glucose energy in the muscles faster. Accordingly, the muscle tissue goes into rigor mortis faster.
Prior art literature teaches that the effects of electrical stimulation works best on slaughtered animals when the nerve tracks of the animal still have the possibility to transfer stimulation. Some prior art methods teach that electrical stimulation has the best effect shortly after death. Other prior art methodologies, such as that disclosed in U.S. Pat. No. 4,561,149, teach that the electric current should be applied to the animal while it is still alive during at least part of the time period following complete stunning of the animal where there has been a complete lost of consciousness, and prior to the clinical death of the animal.
In the prior art methods of applying electrical stimulation to slaughtered animals, electrical stimulation can involve the use of direct current or alternating current, voltages that range between 20-3,600 volts, frequencies that range between 0-60 Hertz, and currents ranging between 0.1-6 amperes. Prior art literature indicates that the later after clinical death the application of electrical stimulation, the more current needs to be applied.
It is believed that the cooling of meat prior to the completion of rigor mortis causes the muscles of the slaughtered animal to contract. The contracting of the muscles causes the meat to be tough. In some cases, contracting or shortening of the muscle tissue may be up to as much as 50%. On the other hand, if the meat of the carcass is not cooled to at least a minimum level, around 65 degrees, prior to the completion of rigor mortis, other problems with the meat will result. One such problem is a condition called pale soft exudative (PSE), which causes the slaughtered meat to be pale in color, soft and watery. PSE results when the meat temperature is too high when the muscle enters rigor mortis. As the meat continues to cool and finally cools to the desired level, it will have a tendency to lose moisture content, be pale in color and be softer than normal.
Most generally, the prior art devices, which apply electrical stimulation to an animal carcass, apply the electrical charge through the entire carcass. An example of such a device and of the conventional method of applying an electrical stimulus to the entire animal carcass is disclosed in U.S. Pat. No. 2,544,861 to Harshan et al. It has been found that using an apparatus such as the disclosed U.S. Pat. No. 2,544,861 to Harshan et al. results in the denser thicker muscled sections of the carcass having PSE tendencies. This results when using an apparatus that applies an electrical stimulus to the entire animal carcass, such as the disclosed in U.S. Pat. No. 2,544,861 to Harshan et al., because the thick muscled sections of the carcass, such as the round and chuck muscles cannot be chilled at the same rate as the less dense loin and rib portions. The inability of the denser meat portions of the carcass to chill as rapidly as the less dense portions, in some instances, provides circumstances for rigor mortis to occur prior to sufficient chilling of the denser meat portions resulting in the denser chuck and round meat portions having the undesirable characteristics of being pale colored, soft and watery.
There is a need for an apparatus and method that allows for the use of electrical stimulation of an animal carcass to improve tenderness and to accelerate the completion of rigor mortis in the fabrication process of an animal carcass, wherein the apparatus and method take the varying densities of the meat portions into consideration.